1. Field of the Invention
This invention relates to direct printing of n and p-type semiconductor materials for use in making thermoelectric coolers.
2. Background of the Prior Art.
Thermoelectric coolers are generally semiconductor devices designed for a medium to low heat pumping capacity requirements. Typical applications include temperature stabilization of bolo meters and ferroelectric detectors, laser diode arrays in fiber optic systems, and for maintaining constant viscosity in ink jet printers. They are generally relatively small devices but nevertheless can generate a temperature differential in the range of 60 degrees Fahrenheit or more. One manufacturer produces square shaped thermoelectric coolers from as small as 4 mm by 4 mm by 2.4 mm high to 13.2 mm by 13.2 mm by 2.2 mm high.
Thermoelectric coolers are typically manufactured by growing single crystals of doped semiconductor materials and then sawing and machining these materials into rectangular shapes with specific crystal orientation, which are then assembled into arrays of n-type and p-type materials. The CRC Handbook of Thermoelectric Coolers edited by D. M. Rowe, CRC Press, Inc., 1995 is a reference on thermoelectric coolers.
Methods of fabrication utilizing the principles of ink-jet printing devices are becoming known in the art. U.S. Pat. No. 6,114,187, Sep. 5, 2000, illustrates the use of an ink-jet printer to prepare a chip scale package which is xe2x80x9cbumpedxe2x80x9d in preparation for making electrical interconnections with pads on a connection surface of the chip. The disclosure of this patent of the same assignee is incorporated herein by reference.
Micro-Jet printing technology, based on ink-jet printing, has been modified in various ways to deposit solder and dielectric polymers in a highly controlled manner on a microscopic scale. Solder and dielectric materials can be applied by the printhead at a high rate of speed controlled by applied voltage pulses at a selected electrical frequency and delay time. This type of printhead is disclosed in U.S. Pat. Nos. 5,193,738, 5,229,016, 5,377,902, 5,643,353 and Patent 5,772,106, Jun. 30, 1998, the disclosures of which are incorporated by reference. The latter U.S. Pat. No., 5,772,106, discloses a printhead useful for liquid metals such as solder.
These same devices can be employed to deposit flowable polymeric materials which can be deposited as droplets from a heated printhead of the above references in the manner of U.S. Pat. Nos. 5,441,679, Mar. 12, 1996; U.S. Pat. No. 5,415,679, May 2, 1995 and U.S. Pat. No. 5,707,684, Jan. 13, 1998 also incorporated by reference. Multiple solder jetting printheads can be arranged to deposit solder or organic dielectric materials as indicated in U.S. Pat. No. 5,686,757 incorporated herein by reference.
It would be desirable if the necessary individual n-type and p-type semiconductor materials could be generated from molten materials and deposited directly at the required location and assembled into thermoelectric coolers without sawing and machining operations.
The present invention discloses a process for fabricating thermoelectric coolers from molten components to produce in situ semiconductor elements which make up thermoelectric coolers. A first substrate is provided having a plurality of spaced apart conductive site pads which comprises the bottom half of a thermoelectric cooler. Molten p-type semiconductor material in a reservoir in communication with a first ejection orifice as held in a digitally driven ejection device and molten n-type semiconductor material having a reservoir in fluid communication with a second ejection orifice is held in a digitally driven ejection device. Because the p-type and n-type semiconductor material is generally molten at an elevated temperature, the digitally driven ejection devices must be capable of holding the molten material in a fluid condition where it can be deposited in droplets, preferably in drop-on-demand mode. The substrate is held at a crystallization temperature which is lower than the freezing temperature of the molten semiconductor materials so that columns of p-type and n-type semiconductor material can be formed on the plurality of spaced apart conductive site pads. A series of droplets of p-type semiconductor material are deposited onto a first conductive site pad on the substrate to freeze it in a column extending away from the first conductive site pad and having a characteristic height terminating at an end. A series of droplets of n-type semiconductor material are deposited onto the first conductive site pad on the substrate to freeze it into a column, spaced from the column of p-type semiconductor material, extending away from the first conductive site pad and having the same characteristic height terminating at an end. The steps are repeated by positioning a second conductive site pad under the first and second ejection orifices and repeating the steps of depositing droplets of p-type semiconductor material and n-type semiconductor material to form separate columns of p-type and n-type semiconductor material which are spaced apart and extending away from the at least a second conductive site pad and having the same characteristic height terminating at their end. Additional columns of p-type and n-type semiconductor material can be formed on any additional conductive site pads until all of the semiconductor elements are formed. A curable or hardenable conductive bonding agent is applied to the ends of the columns of the p-type and n-type semiconductor material. The conductive bonding agent may be a curable conductive epoxy or solder can be used.
A second substrate having a plurality of spaced apart conductive bonding sites including a first conductive bonding site is provided to serve as the top half of the thermoelectric cooler to be produced. These conductive bonding sites have a size and spacing which is similar to that of the conductive site pads, but offset laterally. When the first and second substrates are placed in facing relation, the first conductive bonding site is mated with a column of p-type semiconductor material on the first conductive site pad and the first conductive bonding site is also mated with a column of n-type semiconductor material on the second conductive site pad. This is followed by the step of bonding the mated columns of p-type and n-type semiconductor material in electrical contact with the mated first conductive bonding site to create a package capable of acting as a thermoelectric cooler. Additional bonding sites are mated and bonded similarly with other p-type and n-type elements formed as columns on other conductive site pads on the substrate.
The process may be applied to produce thermoelectric cooler elements from p-type and n-type semiconductor materials having different melting temperatures. The process may be modified in this instance by altering the sequence of steps and depositing the higher freezing temperature semiconductor material before the lower freezing temperature semiconductor material is deposited. The first non-conductive substrate is provided having a plurality of spaced apart conductive site pads. One of p-type or n-type semiconductor material having a higher freezing temperature is held in a digitally driven droplet ejection device having a heated reservoir in fluid communication with a first ejection orifice. The non-conductive substrate is held at a crystallization temperature which is lower than the freezing temperature of the molten semiconductor material having the higher freezing temperature. A column of semiconductor material on each of the plurality of spaced apart conductive site pads on the first non-conductive substrate is formed by depositing droplets of the higher freezing temperature semiconductor material thereon from the first ejection orifice, each column terminating at an end portion and having the same characteristic height.
The other of the molten n-type or p-type semiconductor material is held in a digitally driven droplet ejection device having a heated reservoir in fluid communication with a second ejection orifice. The first non-conductive substrate is held at a crystallization temperature which is lower than the freezing temperature of the other of said n-type or p-type semiconductor material. A separate column of semiconductor material is formed by depositing droplets of the lower freezing temperature semiconductor material on each of the plurality of spaced apart conductive site pads on the first non-conductive substrate wherein each column is a separate column terminating at an end portion and having the same characteristic height as the already formed columns of semiconductor material. A bonding agent is applied to the ends of the columns of p-type and n-type semiconductor material and they are placed in facing relation with one of a plurality of conductive bonding sites on a second non-conductive substrate wherein one or more conductive bonding sites are mated with a column of one of the types of semiconductor material on one conductive site pad and another of the types of semiconductor material on a different conductive site pad. Other mated columns of p-type and n-type semiconductor material are bonded in electrical and physical contact with ones of the plurality of conductive bonding sites to create a package capable of acting as a thermoelectric cooler.
The invention makes it possible to produce in situ columns of n-type and p-type semiconductor materials directly from molten material to make thermoelectric coolers. The process is digitally controlled by a computer controlled system that in combination with an X-Y stage is programmed to move the conductive site pads relative to the orifices from which the molten material is ejected after a columnar element is formed. The substrate is indexed to the next conductive site pad where an additional column of semiconductor material is formed until a complete array of conductive site pads each having the appropriate p-type and n-type columns of semiconductor elements is prepared. The cost of sawing and machining individual pieces is eliminated, waste is eliminated and the process is well suited for automation.